Convert a C3d and TRC files to BVH
You can import .c3d file in blender like this:

https://stackoverflow.com/questions/20499320/how-to-import-c3d-files-into-blender

The problem is it won't be an armature, but animated locations of points:



Some points will be dead in the center (marked).


Adjust the frame range to fit the mocap data
Delete the dead markers
Re-build the armature - add armature to scene


looks like we have markers from both sides for each leg - that means the bones will be somewhere in between markers
Building the armature:


select a marker (or two if I want position in between) and align 3D cursor with 'Shift-S > Cursor to Selected'
then enter edit-mode of armature and select a joint and 'Shift-S > Selection to Cursor'
this way I can place all the bones (joints) precisely


also recalculate the Roll of the bones with 'Ctrl+N > Cursor' with 3D cursor on some reference marker

Name the bones properly
Constraining location of root bones:


use copy_location constraint
or two of them with second on 50% influence to get in-between position of two markers
similar for center of 3 markers, you get the idea

Constraining bones direction


use damped_track constraint
also multiple of them with adjusted influence for center of group of markers




Bake the animation to Action with Pose > Animation 

Bake Action: visual_keying_ON, clear_constraints_ON
Export armature as .bvh from the blender main menu (you might need to enable BVH addon).


These steps will result in having a .c3d file converted to a .bvh file.
A sample .blend file with rigged markers here: